This invention generally relates to apparatus and methods for characterizing the internal pressure of closed containers and more particularly to apparatus and methods that detect and compensate for signal distortions in performing such analyses.
Food, beverage, and drug containers, for example, are often sealed under vacuum (e.g., coffee) or internal pressurization (e.g., beer). If biological contamination or seal failure occurs, the product quality may be significantly degraded and may be dangerous to consumers. Accordingly, manufacturers test the internal pressure/vacuum of containers before shipment to identify and remove defective containers.
Two non-intrusive testing techniques are shown in Hayward, U.S. Pat. No. 3,802,252 and Woringer, U.S. Pat. No. 5,353,631, both of which are assigned to Benthos.TM., Inc., and incorporated herein by reference. Systems of the type described in the foregoing patents have been sold under the name TapTone.TM.. In such systems, a conductive surface of a closed container is vibrated without contacting it. This is accomplished using a pulsed magnetic field, and the resulting sound is analyzed to determine the pressure in the container. A microphone senses the resulting acoustic energy and converts it into an electrical signal. In the Hayward scheme, analog electronics are used to determine whether the signal has a detectable level of energy within a pre-tuned frequency band. If a signal is detected within the band, it is inferred that the can is good. In the Woringer scheme, a similar test is performed using digital signal processing (DSP) electronics and software.
Because closed containers are complex vibratory systems which often exhibit nonlinear effects, it is not uncommon to find in the use of such systems that the acoustic return signals have been modulated by vibratory modes of the container other than the fundamental mode typically used to predict internal pressure. When such distortions are present, the information contained in the acoustic signal has been corrupted by misleading information that can lead to false rejections of containers.
Consequently, it is a primary object of the present invention to provide apparatus and methods for detecting and compensating for the presence of extraneous signal modulation in acoustic signatures used to characterize the internal pressure of containers.
It is another object of the present invention to provide apparatus and methodology for dynamically changing the process control limits for judging internal pressure as a function of slowly varying changes in the upstream container filling process to optimize throughput without permitting the passage of reject containers.
Other objects of the invention and will in part appear hereinafter and will in part be obvious when the following detailed description is read in connection with the drawings.